An eel is a long, snake-like fish that lives in both salt water and fresh water. Unlike most fish, eels have smooth, slippery skin and no visible scales, which makes them incredibly hard to hold onto. They swim by rippling their entire body in waves, moving through water with an almost hypnotic smoothness.

Some eels, like the electric eel (which isn't technically a true eel), can generate powerful electric shocks to stun prey or defend themselves. The moray eel hides in rocky crevices and has a fearsome appearance with sharp teeth, though it typically only bites when threatened. Perhaps most remarkable is the migration of freshwater eels: they're born in the ocean, spend most of their lives in rivers and lakes, then somehow find their way back across thousands of miles of ocean to spawn in the same place they were born.

Calling someone slippery as an eel means they're hard to pin down or catch, either physically or when trying to get a straight answer from them. The phrase captures that frustrating, wriggling quality that makes eels so difficult to grasp.